For a lubricating oil composition used for automatic transmissions, manual transmissions, and internal-combustion engines, improvement in various durability such as thermal oxidative stability, wear resistance, fatigue prevention property, improvement in viscosity-temperature characteristics to improve fuel-saving performance, and improvement in low temperature viscosity characteristics such as reducing of viscosity in a low temperature and improvement of low temperature fluidity have been required. For the sake of such performance improvements, reformation of low temperature viscosity characteristics of base oil such as increasing viscosity index of the base oil has been attempted. With such a base oil, various additive agents such as antioxidants, cleaning dispersants, antiwear agents, friction modifiers, seal swelling agents, viscosity index improvers, antifoam agents, and coloring agents have been adequately combined to make a lubricating oil composition.
Currently, as a method to reform the viscosity-temperature characteristics for the purpose of improving fuel-saving performance, reducing viscosity of base oil at the same time increasing amount of the viscosity index improver has been generally carried out. However, in a case where the viscosity of base oil is reduced, the viscosity index improver is degraded by being subjected to shear, whereby thickening property of the viscosity index improver is impaired. As a result of this, whole viscosity of lubricating oil composition is reduced. Therefore, problems such as degradation of lubrication performance (lubricity) over time and shortage of oil pressure over time have been concerned.
Against these problems, in order to support the fuel-saving performance and the lubricity at the same time, improvement in viscosity characteristics of lubricating oil composition by combination use of a base oil having a high viscosity or by using a base oil having a good low temperature characteristics has been suggested. Also, improvement in lubricity and fatigue life of lubricating oil by adequately adding a phosphorus-based extreme pressure agent, sulfur-based extreme pressure agent and the like has been suggested (see Patent Documents 1 to 3).
However, as the performance of the lubricating oil has been further demanded, such improvements in fuel-saving performance and shear stability that can be realized by these conventionally-known method are becoming insufficient to the demand level to meet. Therefore, a development of a lubricating oil composition in which both of the fuel-saving performance and the shear stability are further improved has been required.